The present invention relates to a method for fixing a spring package to a top nozzle in a fuel assembly of a nuclear power reactor.
FIG. 1 shows a known fuel assembly 1 for a pressurized-water reactor. The fuel assembly 1 contains fuel rods 2 and guide tubes 3 retained into a bundle by the spacers 4. The bundle is placed between a bottom nozzle 5 and a top nozzle 6. The top nozzle is provided with springs in the form of spring packages 7 for holding the fuel assembly 1 pressed against the bottom in the reactor core. The spring package 7 is fixed to the top nozzle 6 by means of a clamp 8 and a screw 9. FIGS. 2 and 3 show in more detail how the spring package 7 has been fixed to the top nozzle 6. FIG. 3 shows that the clamp 8 itself is fixed to the top nozzle 6 by means of a screw 10. FIG. 4 shows how the device is intended to operate. On the left, the fuel assembly 1 is positioned on the lower core plate 11. On the right, the fuel assembly 1 is shown when the upper core plate 12 has been mounted. In this latter position, the spring package 7 acts against the core plate 12 and retains the fuel assembly 1 in position in spite of a great upward flow of coolant in the core. Occasionally, however, the screw joint 9, 10 has broken. This has caused screw heads and spring packages 7 to loosen from the top nozzle 6 and caused the insertion of the control rods of the reactor to become blocked, thus jeopardizing the shutdown of the reactor. When the spring packages 7 loosen, the heavy upward coolant flow in the reactor core causes the fuel assembly 1 to lift. This may cause damage to the control rods and to the upper and lower core plates of the reactor.